SquirrelMail configuration is file based. If you've installed SquirrelMail directly from source you'll find the configuration file in config/config.php, but if you're using a SquirrelMail package from your OS distribution the actual configuration file may be located somewhere else (such as /etc/) - usually with a symbolic link from config/config.php. Read the documentation that came with the distribution package to find out more about OS distribution specific details. The rest of these instuctions assumes that you've installed SquirrelMail from source.